Age hardening austenitic steel



United States Patent 2,706,696 AGE HARDENING AUSTENITIC STEEL Peter Payson, New York, N. Y., assignor to Crucible Steel Company of America, New York, N. Y., a corporation of New Jersey No Drawing. Application April 24, 1951, Serial No. 222,736

6 Claims. (Cl. 148-31) This invention pertains to heat and corrosion resisting, austenitic steels which are ageor precipitation-hardenable. The invention pertains more especially to an essentially straight manganese-nickel-chromium steel of relatively high carbon content, which is precipitation hardenable to at least C 32 Rockwell and up to about C 50 Rockwell or more for certain analyses, and which requires no additions of other alloying elements to achieve this result. The invention also pertains to a process of precipitation hardening this steel.

Austenitic manganese-nickel-chromium steels of certain analyses have been known for many years, and have proved especially useful for parts requiring corrosion and heat resistant properties as well as strength at elevated temperatures, and for parts in which very low magnetic permeability is desired. Also, austenitic steels have been used in some applications where relatively high coefiicient of expansion is desired. Austenitic steels are relatively easy to fabricate both hot and cold, and, therefore, they have been produced and used in many wrought forms, as well as in the form of castings.

One noteworthy short coming of the known types of austenitic manganese-nickel-chromium steels is that they have not been appreciably hardenable by heat treatment, and, therefore, can be used generally with only relatively low stresses. Certain analyses are hardenable by cold working, and large quantities of such steels in the form of cold rolled strip have been utilized for constructional parts where strength combined with corrosion resistance is required. But many constructional parts cannot be made from cold roll-ed sections and there has accordingly been a need for an austenitic steel which can be hardened in any size or shape by a relatively simple heat treatment.

The hardening of ordinary steels, such as straight carbon or essentially ferritic alloy steels, involves a heating to a sufficiently high temperature to form austenite, followed by a cooling procedure which permits the austenite to transform to the relatively hard transformation products known as martensite or bainite. Generally, the temperature at which the transformation to the hard products occurs during cooling is between about 600 and 80 F. In some fairly high alloy steels, the austenite may be retained substantially untransformed down to room temperature, and may be made to transform subsequently to the aforesaid hard transformation products, either by refrigeration to very low temperatures, that is, below room temperature and down to about 300 Fahrenheit below zero, or even lower, or by a conditioning treatment which consists of a heating to a temperature generally between about 1000 and 1400 F. for several hours followed by a cooling to room temperature, or by a combination of both. In any event, the transformation of the austenite to said relatively hard products is accompanied by a change from the substantially nonmagnetic condition of the steel to a magnetic condition, since the aforesaid products of the transformation of austenite are magnetic.

Another type of hardening by heat treatment of certain high alloy steels containing chromium and either or both nickel and manganese together with other alloy additions, depends on the transformation of ferrite which is magnetic, to sigma which is substantially non-magnetic. In this type of hardening, designated heat hardening in my U. S. Patents Re. 20,421; 2,051,415; 2,141,016; 2,484,903; and 2,518,715; there is a change from a magnetic condition in the prehardened state of the steel to a substantially non-magnetic condition in the hardened state of the steel.

There is still another method of hardening by heat treatment certain other high alloy steels containing manganese, nickel and chromium together with other alloy additions, which is designated precipitation, or age hardening. This consists generally of a two stage treatment in which there is first a heating at a relatively high temperature to dissolve a compound within the austenitic matrix of the steel, followed by a quench to retain the compound in supersaturated solution, and then a heating at a lower temperature than the first to precipitate the compound in the matrix of the steel. During this treatment there is substantially no change in the atomic structure of the matrix. That is to say, the austenitic matrix maintains its face-centered cubic lattice throughout all steps of the treatment.

In the precipitation, or age hardenable austenitic manganese, nickel, chromium containing steels previously known, it has been necessary to introduce therein, compound forming elements other than those ordinarily found in these steels, that is, other than carbon, manganese, phosphorus, sulfur, silicon, nickel, chromium, and residual amounts of other elements. That is to say, it has been considered necessary, for imparting precipitation hardening properties to such steels, to introduce therein, compound forming elements, such for example, as aluminum, copper, molybdenum, titanium, nitrogen, etc. Thus in the invention of my U. S. Patent 2,523,917, there is added about 2.5 to 4% of aluminum to a steel containing carbon, manganese, molybdenum and chromium, within specified limits, for producing an age hardening, austenitic steel. Again, according to the invention of my application Serial No. 120,412, filed October 8, 1949, now U. S. Patent No. 2,561,945 there is added about 3 to 5% of aluminum to a steel containing relatively high nickel, together with manganese and chromium, within specified limits for producing an age hardening, austenitic steel. Also, according to the invention of my application Serial No. 63,200, filed November 23, 1948, now U. S. Patent No. 2,587,613, I produce an age hardening, austenitic steel by additions of about 3 to 15% in aggregate of tungsten and molybdenum, to a steel containing large amounts of nickel and chromium. Other investigators have employed sizable additions of titanium or beryllium to produce age1 hardenable, austenitic chromium-nickel-manganese stee s.

Now I have discovered that within certain limits of carbon and chromium, in austenitic steels containing in addition, manganese and nickel, within certain limits, but containing no other elements aside from incidental amounts of phosphorus, sulfur, silicon and nitrogen, I can produce age hardening to an appreciable degree, that is, to develop a hardness of upward of 300 Brinell (32 Rockwell C), by the employment of appropriate solution and aging temperatures. The critical limits of composition, as will be shown below, are those which establish: (1) the degree to which the steel will harden; (2) the stability of the austenite against transformation during the age hardening of the steel; and (3) the formation of a substantially austenitic structure at the solution temperature.

Some small amount of hardening may be encountered in ordinary austenitic stainless steels as commercially produced when the steels are held for some time at about 1100 to 1400 F. For example, it has been reported in a recent publication by Smith, Dulis, and Houston, Trans. ASM, vol. 42, 1950, that the low C, 18% Cr, 10% Ni steel, Type 304, has a hardness of 142 Brinell as solution treated, and 150 Brinell after 3,000 hours at 1100 F.; that the 18% Cr, 12% Ni, 2.5% Mo steel, Type 316, increases in hardness from 145 Brinell as solution treated, to 190 Brinell after 3,000 hours at 1300 F.', and that the 18% Cr, 11% Ni, 0.4% Ti steel, Type 321, increases from 149 Brinell to 215 Brinell after 3,000 hours at 1300 F. But age hardening to this slight degree, is of no commercial significance. In sharp contrast to the above, the chromium-nickel-manganese steels within the range of analysis of my invention are age hardenable to at least 300 Brinell, i. e. C 32 Rockwell, and up to about C 50 Rockwell for certain analyses.

The broad range of analysis of steel in accordance with my invention, which is age hardenable in the magnitude aforesaid, is that containing about: 0.4 to 1.5% carbon, 0.1 to 25% manganese, up to 25% nickel, with the nickel and maganese contents aggregated 8 to 28%, 12 to 30% chromium, up to silicon, and the balance substantially all iron, except for the usual residuals within commercial tolerances, such as phosphorus, sulfur, etc. A preferred range of analysis is that containing about: 0.45 to 0.8% carbon; 1 to manganese; 1 to nickel; with the sum of nickel and manganese limited to about 9 to 28%; 15 to chromium; 0.05 to 3.5% silicon, and the balance substantially all iron as aforesaid.

More specific ranges of analysis which undergo optimum age hardening, are as follows:

1 Analysis, Percent by Weight C Mn Si Ni Gr Bal.

a 1 i a Fe 11 4 Fe 45 1.5 7. 15. m "l g gal It "l? a With respect to each of the aforesaid analysis ranges, the aggregate content of phosphorus and sulfur should not exceed 0.3%. Aluminum where present should not exceed 0.3%. Likewise molybdenum and copper when present should not exceed 0.5%, each, while nitrogen should not exceed 0.15

That carbon is an important element in this age hardening mechanism of the steel of the present invention is shown in Table I below.

TABLE I Efieet of carbon content on age hardening of austenitic Mn-Ni-Cr steels [All samples quenched from 2,300 F. and aged for 16 hours at1,200; 1,300; and 1,400" F.]

- Rockwell 0" Hardness Alter Analysis, Percent--Bal. Fe Aging Bar Quenched hours hours hours 1. 4 5 12. 3 19. 0 13 18 23 21 1.2 4 l2. 7 19. 6 8 20 28 23 l. 3 5 l2. 5 19.0 18 28 36 31 1. 3 5 l2. 5 19. 7 13 27 1. 4 0 12. 2 19.4 19 30 39 33 l. 4 5 12.5 19. 7 23 30 39 34 1. 5 6 12. 4 19. 7 24 32 38 31 No'rr:.All above samples are practically non-magnetic.

These data show that although appreciable hardening occurs in the 30% C and 38% C steels, the hardness attained after 16 hours aging is less than 300 Brinell or C 32 Rockwell. I, therefore, set a lower limit of 0.40% carbon in the composition of the age hardenable austenitic Mn-Ni-Cr steels of this invention. When the carbon is much in excess of about 1.0% the steel is difiicult to forge and therefore for wrought steel I limit the carbon to about 1.0% maximum. However, for castings I may use as much as 1.5% carbon in these steels.

To develop minimum hardness of 32 C Rockwell, age

hardenable austenitic Mn-Ni-Cr steel must contain a minimum of about 12% Cr as shown in Table II.

TABLE II Efieet of chromium content on age hardening of Mn-Ni-Cr steel [All samples quenched from 2,300 F. and aged for 16 hours at 1,200

1,300; and 1,400 Analysis, Pereent-Bal. Fe Rockwell lg i gih f After Bar 0 M s N 0 AS 1 i 3 i 5 6 n i i r 1 Quenched hours hours hours 3217- 55 1. 2 5 12. 3 5. 2 12 14 16 17 3197v 49 1. 3 5 12. 3 9. 8 1G 18 29 24 3198. 52 l. 4 6 12. 5 14. 7 17 26 3G 30 3294. 48 1. 2 .5 12.3 17. 6 11 30 33 27 3199 57 1. 4 6 12. 2 19. 4 19 3O 39 33 3295. 54 1. 2 5 12. 2 22. 6 16 31 34 31 3200. 5G 1. 3 6 12. 5 24. 1 21 34 36 30 Nora-All above samples are practically non-magnetic except those of Bar 3217 which are slightly magnetic. These data indicate no particular advantage in regard to age hardening for steels with chromium over about 19%, but higher chromium is desirable for maximum resistance to corrosion, and to oxidation at high temperatures, and for special applications, therefore, I may use up to about 30% Cr.

In order to maintain an austenitic structure in my pre cipitation hardening Mn-Ni-Cr steels it is necessary to hold the Ni content at a relatively high level, or to bolster the austenite stabilizing effect of Ni by means of relatively high Mn content. The data on the effects of variations in Ni and Mn are shown in Table III. The criterion for the stability of the austenite is a relatively simple magnetic test consisting of touching the sample under test with a good Alnico horseshoe magnet, and noting the relative response of the sample to the magnet.

TABLE III Effect of manganese and nickel content on age hardening of austenitic Mn-Ni-Cr steels [All samples quenched from 2,300 F. and aged for 16 hours at 1,200"; 1,300; and 1,400 F. Each sample tested for hardness and magnetic susceptibility.]

Rockwell C Hardness, After Analysis, Percent Bal. Fe Aging Bar 0 M s N 0 AS 1 i 3 95 1 Quenched hours hours hours 2. 3 5 2 19.7 24 26 43 "50 4. 7 5 2 l9. 8 22 33 42 47 7. 7 5 2 18. 7 22 33 41 *39 9. 7 6 2 19. 6 25 37 40 37 13. 7 6 2 19.7 23 33 41 37 1. 3 5 2. 2 l9. 1 22 20 48 49 4. 5 4 2. 3 19. 4 17 28 38 "*39 7. 9 4 2. 3 19. 3 19 31 39 36 1. 3 5 4. 8 19. 5 17 29 *39 40 4. 6 5 4. 8 l9. 6 19 38 40 36 1. 2 5 7. 5 l9. 5 15 28 33 *32 1. 4 5 7. 4 l9. 2 21 28 40 37 l. 3 5 7. 5 l9. 6 17 37 40 36 4. 7 6 7. 0 19. 8 22 36 40 35 1. 4 G 12. 2 19. 4 19 30 39 33 4. 8 6 13.3 20. 5 21 31 36 32 1. 3 5 l7. 5 19. 6 20 34 3G 31 1. 3 6 22. 5 l9. 6 19 28 33 26 1. 3 6 27. 5 19. 7 18 30 30 29 NOTE.A11 samples in above table practically non-magnetic except those marked which are slightly magnetic, and those marked which are definitely magnetic.

As a criterion of the stability of the austenite, I consider the magnetic susceptibility of the steel after the 16 hour agings at 1300 and 1400 F. The stability is considered adequate if the steel is practically non-magnetic after the 1300 F. aging, and no more than slightly magnetic after the l400 G. aging. On this basis I establis for my age hardening austenitic steels a minimum manganese plus nickel content of about 8%, and increase this giilnimum sum when the Cr in the steel is high as stated e ow.

There is another consideration in the production of When the chromium is over 19%, I use a minimum of 1.0% nickel and increase the nickel gradually at the rate of 0.5% for each 1% increase in chromium. For example, when the chromium in the steel is 25%, I use a minimum of 4% nickel; when the chromium is 30%, I use a minimum of 6.5% nickel. Furthermore, as the chromium increases above 23%, I increase the minimum sum of manganese plus nickel to 9.0% and when the chromium is over 25% I increase the minimum sum of the manganese plus nickel to 10%, to assure that the steel is substantially austenitic as solution treated and that the austenite is adequately stable during the aging treatment.

The higher the manganese plus nickel content, the more stable the austenite against transformation after very long exposures at 1100 to 1400 F. For parts that are to be exposed for very long times at these temperatures, for example, wheels for steam or gas turbines for marine or stationary power plants, I may use up to about 25% of either or both manganese and nickel. However, as may be seen in the hardness values of the last four analyses given in Table III, the maximum attainable hardness decreases fairly gradually as the sum of manganese plus nickel increases above about 18%, until when the sum is about 29%, the attainable hardness is under 300 Brinell, that is, Rockwell C 32.

Silicon in amounts ordinarily found in austentic steels has practically no elfect on the hardening of my age hardenable steels as may be seen in Table IV.

TABLE IV Eflect of silicon content on age hardening of austenitic Mn-Ni-Cr steels [All samples quenched from 2,300" F. and aged at 1,200; l,300 and 1,400 F. for 16 hours] Analysis, Percent-Bel. Fe gggi g zgfgi Bar 0 Mn s N c As a l? 1 1? Quenched hours hours hours Nora-All above samples are practically non-magnetic.

much as 5.0%. Amounts over about 1.5% effectively improve the oxidation resistance of the steel when it is used in elevated temperature service. For wrought materials I generally limit the silicon to amounts up to about 5 3% since the forgeability of the steel becomes successively poorer as the silicon is increased beyond this amount. However, for castings I may use larger amounts, up to about 5%.

Although I have used 2300 F. for the solution temperature to illustrate the age hardening of the various steels shown in Tables I to IV inclusive, I am not restricted to this single solution temperature for all the age hardenable austenitic Mn-Ni-Cr steels of the invention. Some of my steels particularly those containing over about 4% manganese attain hardnesses over 300 Brinell when aged after solution treatments as low as 2200 F., and even lower, as may be seen in Table V below. On the other hand, in some steels, appreciable hardness, that is, a minimum of Rockwell C 32, is not attained unless a solution temperature around 2300 F. is used.

The optimum aging temperature is about 1300" F., and this aging temperature was used to show the effect of solution temperature in Table V.

25 TABLE V Samples quenched from indicated solution temperatures and aged for 16 hours at 1,300 F.]

Analysis, Percent-Bah Fe Rockwell "0 Hardness Bar 2,100 F. 2,200 F. 2,s00 F.

0 Mn Si N1 Or A A A s s s Q Aged Q Aged Q Aged N 0TE.AI1 above samples are practically non-magnetic.

Although the hardness attained by an aging at 1400 F. is generally much lower than that attained by an aging at 1300 F once my age hardenable Mn-Ni-Cr austenitic steels may have application for constructional parts to be used at room temperature where abrasion resistance and good strength are required under corrosive conditions, it is probable that the most important application of these steels will be for high temperature service where resistance to rupture under high stress conditions is required together with oxidation resistance.

TABLE VI Resistance to averaging in Mn-Ni-Cr age hardenable austenitic steels solution treated at 2,300 F. and aged at 1,300 F.

Analysis, PercentBal. Fe Rockwell 0" Hardness, Reheated cumulatively Bar 0 1,400 1,500 1,600 1,700 1,800 0 Mn s1 Ni Cr if F. 2 F. 2 F. 2 F. 2 F. 2 hours hours hours hours hours It has been established for example that Bar 3206, the analysis of which appears in Tables III, V and VI, when solution treated at 2150 F. and aged at 1300" F. for 16 hours to a hardness of 36 C Rockwell has a rupture life of 1000 hours at 1200 F. under a stress of 35,000 p. s. i. Under the same conditions of testing, conventional Ni-Cr austenitic steels, Types 304; 347; and 316; would rupture after only about to 80 hours, respectively, at 35,000 p. s. i. The maximum stresses these conventional steels could endure for 1000 hours at 1200 F. Without rupture are about 14,000; 26,000; and 23,000 p. s. i., respectively. Thus it may be seen that the steel of this invention is appreciably stronger at 1200 F. than conventional steels of approximately the same percentage of alloying elements and at the same time contains none of the elements columbium, molybdenum, tungsten, or cobalt, which are in short supply.

Another application of importance for which my austenitic age-hardenable Mn-Ni-Cr steels are eminently qualified is the exhaust valve of automotive engines for which good strength is required so that the valve will resist stretching at temperatures up to 1600 F. with stresses up to about 10,000 p. s. i.; as well as good resistance to corrosion in combustion products of gasoline containing lead compounds. It has been established that one of the steels of this invention analysing:

Bar 0 Mn Si Ni 01 When solution treated at 2250 F. and aged at 1300 F. for 16 hours to a hardness of C 39 Rockwell stretched about 0.5% in 22 hours at 1500 F. with a stress of 18,000 p. s. i. and about 0.5% in 18 hours at 1600 F. with a stress of 12,000 p. s. i. For comparison, the best of the conventional austenitic automotive exhaust valve steels analysing:

Bar

stretched 0.5 in only 4 hours at 1500 F. with a stress of 15,000 p. s. i.; and stretched 0.5% in only 4 hours at 1600 F. with a stress of only 10,000 p. s. i. Furthermore, the resistance of the age hardenable, austenitic Mn-Ni-Cr steel, Bar 3311, of this invention, to corrosion in molten lead oxide at 1675 F. was much superior to that of the conventional austenitic exhaust valve steel, Bar 2689X, being about 2.3 gms. per sq. in. per hour against 4.5 gms. per sq. in. per hour. Thus on the basis of stretch resistance and corrosion resistance the steel of this invention is markedly superior to the best of the conventional automotive exhaust valve steels. Furthermore, the steel of this invention can be hardened by heat treatment so that it will have adequate resistance to wear at the tappet end, and along the stern of the valve, whereas the conventional austenitic steel exhaust valve must have a stem of a hardenable steel welded to the head of the valve, which entails a more complex valve fabricating procedure than the manufacture of a one piece valve such as can be made from the steel of this invention.

Excellent steels in accordance with the invention as regards oxidation and corrosion resistance as well as elevated temperature strength, are those containing upwards of 0.4% silicon.

I claim:

1. An austenitic alloy steel which has been precipitation hardened to at least C 32 Rockwell by solution treating at about 22002300 F. and quenched and thereupon aged at about 1100-1400 F., said alloy having substantially the following composition: 0.4 to 1.5% carbon; 12 to 25% chromium; 0 to 25% nickel; 0.1 to 25 manganese, the combined nickel and manganese content comprising 8 to 28%; 0 to 5% silicon; up to 0.3% aluminum; up to 0.5 each of molybdenum and copper; balance iron.

2. An austenitic alloy steel which has been precipitation hardened to at least C 32 Rockwell by solution treating at about 22002300 F. and quenched and thereupon aged at about 11001400 F., said alloy having substantially the following composition: 0.45 to 0.80% carbon, 15 to 25% chromium, 1 to 20% nickel, 1 to 10% manganese, the combined nickel and manganese content comprising 9 to 28%, 0.05 to 3.5% silicon, balance substantially iron.

3. An austenitic alloy steel which has been precipitation hardened to at least C 32 Rockwell by solution treating at about 22002300 F. and quenched and thereupon aged at about 11001400 F., said alloy having substantially the following composition: 0.45 to 0.7% carbon, 15 to 25% chromium, 2 to 4% nickel, 8 to 25 manganese, the combined nickel and manganese content comprising 10 to 29%, 0.1. to 1.0% silicon, balance substantially iron.

4. An austenitic alloy steel which has been precipitation hardened to at least C 32 Rockwell by solution treating at about 22002300 F. and quenched and thereupon aged at about 11001400 F., said alloy having substantially the following composition: 0.45 to 0.7% carbon, 20 to 23% chromium, 2.5 to 4.5% nickel, 5.5 to 7.5% manganese, the combined nickel and manganese content comprising 8 to 12%, 0.1 to 2.5% silicon, balance substantially iron.

5. An austenitic alloy steel which has been precipitation hardened to at least C 32 Rockwell by solution treating at about 22002300 F. and quenched and thereupon aged at about 1100-1400 F., said alloy having substantially the following composition: 0.45 to 1.2% carbon, 15 to 25% chromium, 7 to 20% nickel, 1.5 to 4% manganese, the combined nickel and manganese content comprising 8.5 to 24%, 0.3 to 3% silicon, balance substantially iron.

6. An austenitic alloy steel Which has been precipitation hardened to at least C 32 Rockwell by solution treating at about 2200-2300 F. and quenched and thereupon aged at about 1100-1400 F., said alloy having substantially the following composition: 0.45 to 0.7% carbon, 18 to 25% chromium, 12 to 25 nickel, 0.5 to 4% manganese, the combined nickel and manganese content comprising 12.5 to 29%, 0.3 to 3.5% silicon, balance substantially iron.

References Cited in the file of this patent UNITED STATES PATENTS 2,225,730 Armstrong Dec. 24, 1940 2,380,821 Breeler et al. July 31, 1945 2,495,731 Jennings July 31, 1950 2,518,715 Payson et al. Aug. 15, 1950 2,561,945 Payson July 24, 1951 OTHER REFERENCES Transactions, American Society for Metals, vol. 32, pages 414 and 415, 1944 edition. Published by the Society, Cleveland, Ohio. 

1. AN AUSTENTIC ALLOY STEEL WHICH HAS BEEN PRECIPITATION HARDENED TO AT LEAST "C" 32 ROCKWELL BY SOLUTION TREATING AT ABOUT 2200-2300* F. AND QUENCHED AND THEREUPON AGED AT ABOUT 1100-1400* F., SAID ALLOY HAVING SUBSTANTIALLY THE FOLLOWING COMPOSITION: 0.4 TO 1.5% CARBON; 12 TO 25% CHROMIUM; 0 TO 25% NICKEL; 0.1 TO 25% MANGANESE, THE COMBINED NICKEL AND MANGANESE CONTENT COMPRISING 8 TO 28%; 0 TO 5% SILICON; UP TO 0.3% ALUMINUM; UP TO 0.5% EACH OF MOLYBDENUM AND COPPER; BALANCE IRON. 